Dumb piezo tweeter & hi-pass question

Well my question's rather simple. I have a cheap Motorola piezo installed on one of my old Peavey TKO combos. The thing is I put a 2.2uF capacitor in order to get rid of low frequencies. I know I could have put the piezo on its own but I didn't like (a little harshy) it so I put the 2.2uF thingy to smooth it out a bit. And it worked and I don't have any problems result-wise. I'm just curious about the theory behind this (sorry guys, I have no electronics background at all). At 8ohms, the hi-pass cutoff frequency is around 7khz. But reading some specifications for such tweeter I found it says that the impedance is 1000 ohms. This isn't varying the total impedance of the system much as...

(8*1000)/(8+1000) = 7.94 ohms

I took the 2.2 uF value from some table, to cut around 7khz when run in 8 ohms. My question is, the capacitor for a desired cutoff frequency should be calculated in relation to the system's impedance or to the particular speaker's impedance (the one of the speaker we want to hi-pass). Thanks so much!

You should calculate it using the particular speaker's impedance. Piezos are tricky, because their impedance changes drastically with frequency. (well, normal drivers' can too, but not so much). Typically, a piezo's impedance rises as frequency gets lower, so they have a built-in natural crossover effect. However that makes designing a crossover for one a bit tricky. In effect, the piezo acts like it already has a capacitor in series with it. And adding another capacitor in series with it again actually lowers the overal capacitance!

You should calculate it using the particular speaker's impedance. Piezos are tricky, because their impedance changes drastically with frequency. (well, normal drivers' can too, but not so much). Typically, a piezo's impedance rises as frequency gets lower, so they have a built-in natural crossover effect. However that makes designing a crossover for one a bit tricky. In effect, the piezo acts like it already has a capacitor in series with it. And adding another capacitor in series with it again actually lowers the overal capacitance!

Click to expand...

Ok, so to clear things up, if the capacitance decreases, the cutoff frequency is increased so that's what I did. Am I right? Thanks so much!

I read something about this a couple of months ago, but not on this forum, and the point that was being made was that piezos sound smoother if you use resistors as well as capacitors in their crossover. Here is one link on the subject:http://www.audioasylum.com/scripts/t.pl?f=pro&m=9396
I think the info was on a site linked to whatever CTS/Motorola has become. Does that ring any bells with anyone?

Using a capacitor in series with a piezo does just about nothing, as the piezo is a high impedance capacitive load and a capacitor will only affect the frequency response of a low impedance resistive load. For more information on piezos go to http://www.partsexpress.com/pdf/piezo.pdf